1. Field of the Invention
This invention relates generally to the production of uniformly sized granular products and, more particularly, to the formation of granules from raw fertilizer product.
2. Prior Art
It has become a standard in the fertilizer industry that dry, solid fertilizer materials be in granular form. This requirement has come about primarily because raw product fertilizers have a tendency to cake and create dust. Caking is wasteful and presents difficulty in handling. Dusts are also wasteful and can create a pollution hazard. Also, in order to accurately and efficiently mix nutrients, uniform granulation is essential.
In the past, two methods have generally been employed for dry formation of granules: (1) crystallization, or (2) compaction. This is especially true for certain desirable primary fertilizer nutrients, such as muriate of potash or sulfate of potash. The crystallization process is extremely expensive and requires that the product be dried once it is crystalized. The compaction process is less costly than crystallization, but remains very expensive. The compaction process requires large expenditures for energy, maintenance and, in many instances, for additives. The compaction process also yields granulated products which are prone to decrepitation, moisture affinity and dusting.
Certain fertilizer materials such as ammonium sulfate cannot be granulated satisfactorily or economically on a commercial scale by compaction. Recent attempts to do so have failed because of high pressure requirements, inability to maintain the compactors and poor product quality (tendency to decrepitate, pick up moisture and cake).
This result has left crystallization as the only commercially acceptable solution for ammonium sulfate granulation. The crystallization process, as stated above, is capital intensive and requires expensive follow-up drying operations. The crystallization process has also resulted in the formation of a requisite amount of fine particles which heretofore have been discarded due to the inability to economically transform them into a saleable granular form.
The use of additives to aid in the granulation process has resulted in increased product costs as well as dilution of the product. Also, "wet" granulation techniques are required when additives are introduced, thus also requiring expensive drying operations. Poor product quality has resulted from the use of "wet" techniques. More particularly, the product is subject to decrepitation during transferring operations and in humid environments.
Some attempts at thermal granulation have been made. These attempts have involved the use of rotary drums, kilns, paddle shaft pugging devices, blungers, and the like. These attempts, as those listed previously, have failed to produce an economically feasible quality product.
Examples of prior attempts at granulation may be found in U.S. Pat. Nos. 3,362,989 and 2,944,878. These patents utilize sublimation routes in order to obtain crystal modifications.
All of the above mentioned processes require that the product be passed through a screening process for classification of acceptable granules of a uniform size and rejection of all others. This is an additional expense, requiring additional equipment and wasting a significant amount of product.